What are matrix expression and vector expresssion classes in Boost.uBLAS?

Question

I have tried reading Boost Documentation, but I don't understand this expression concept. Matrix class inherits a matrix_expression class, I don't understand what is this class used for.

Thank You.

Answer 1

It is a mechanism to contain behaviour of some kind.

A matrix_container is - as the word says - a container.

A matrix_expression is an expression ( like the addition of two matrices ). It is based on the RCTI idiom and helps the design of the library. As an example, say you want to write the assignment to a class object (member function). Assume a simpler context than ublas, where you only have a matrix type and you want to write:

template <typename _E>
void assign( _E const & e );

The problem with this is that, then you could assign a socket or an std::string to a matrix. Typically, one tries to avoid this ;-)

So, a better way is to write :

template <typename _E>
void assign( matrix_expression<_E> const & e ) ;

Now you can only assign matrix expressions. I emphasize that this is only the logic and not the ublas specific design - however, the logic is always this one.

Say now you wanted to write your own operator *= with two matrices, or with a matrix and an expression or with two expressions and maybe for *= matrix you want to use lapack. The distinction between matrix_expression and matrix_container would help you write specialized versions of this operation.

PS: I emphasize, this is the underlying logic/motivation not what actually happens. If you ever delve deeper into the ublas guts, you will see more of how the same logic is employed.